Oilpan: Remove guardRef and guardDeref from TreeScope.

Source/core/page/DOMSelection.cpp

 /*
  * Copyright (C) 2007, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 50 matching lines @@
 
 DOMSelection::DOMSelection(const TreeScope* treeScope)
     : DOMWindowProperty(treeScope->rootNode().document().frame())
     , m_treeScope(treeScope)
 {
     ScriptWrappable::init(this);
 }
 
 void DOMSelection::clearTreeScope()
 {
-    m_treeScope = 0;
+    m_treeScope = nullptr;
 }
 
 const VisibleSelection& DOMSelection::visibleSelection() const
 {
     ASSERT(m_frame);
     return m_frame->selection().selection();
 }
 
 static Position anchorPosition(const VisibleSelection& selection)
 {
@@ skipping 468 matching lines @@
     return node->document() == m_frame->document();
 }
 
 void DOMSelection::addConsoleError(const String& message)
 {
     if (m_treeScope)
         m_treeScope->document().addConsoleMessage(JSMessageSource, ErrorMessageLevel, message);
 }
 
 } // namespace WebCore